This invention relates to optoelectronic sensors as well as a method for detecting objects with light beams emitted by a light source and, following reflection, received by a light receiver.
Optoelectronic sensors are often used for detecting the intrusion of objects into monitored regions. They are used in a variety of ways, from motion detectors via burglary protection to securing automatic doors. The detection of objects is also of importance in the automation industry.
A reflection light barrier is an example of an object detecting sensor which, generally speaking, has a light source, such as an LED, that emits a beam which strikes a retroreflector at the opposite end of the monitored region. The reflector returns the light, and the reflected beam is then detected by a light receiver arranged proximate the light source. When the light beam is interrupted, the light receiver receives nothing and initiates a deactivation function or signal.
Emitted and reflected light are directed through beam shaping optics. When the optics for the emitted and reflected beams is the same, it is referred to as auto collimation. Alternatively, a separate lens can be provided for the light source and the light receiver according to the double-eye principle wherein the two lenses are arranged in close proximity to each other.
An erroneous signal or deactivation function can be encountered with light barriers when a light reflecting or very bright object enters the light transmission path from the source to the retroreflector and/or the light reflecting path from the retroreflector to the light receiver. Such an object can generate so much light that the light barrier is no longer able to recognize an interruption of the light beam. Such interruptions are sometimes also referred to as mirror or white light security.
Conventionally, such problems are solved with polarized light. For this, the emitted beam is polarized. The retroreflector maintains the polarization, and a polarization filter is arranged in front of the light receiver, which is optically crossed relative to the polarizer, and therefore permits unimpeded passage of the received light beam. However, when the emitted light beam strikes a mirror or a bright object, the light will be polarized in the wrong direction or loses its polarization altogether. As a result, the received light beam is overpowered by the light from the mirror and/or a bright object so that the mirror or bright object is interpreted as an interruption of the light beam.
It is also known to use a physical beam splitter for reflective light barriers, such as a metalized glass or plastic substrate, for separating the emitted light beam from the reflected beam. These are normally 50:50 beam splitter plates with glass surfaces of high optical quality onto which thin metal layers are deposited to effect partial reflections.
However, the use of such 50:50 beam splitters has a number of disadvantages and problems. Relative to the overall cost of a reflection light barrier, the beam splitter constitutes a high-value optical component that is costly. Since the beam splitter causes a 50% loss of light intensity in the emitted light beam, the light spot on an object to be detected or on the retroreflector is weak and difficult to see. This makes it substantially more difficult to manually align the light barrier. Since the signal detected by the light receiver is twice reduced by 50%, the maximal possible reach of the light barrier is relatively short.
Light remitted by a mirror or a bright object also includes a light component that is properly oriented for the polarization filter. As a result, very bright or highly reflective objects can erroneously provide a receiving signal. Reflection light barriers are therefore not entirely secure against mirror or white light interferences.
In addition, 50% is a significant portion of the emitted light which must be internally absorbed by directing it to a tubular cavity or the like of the light barrier housing where it must be diffused. Such light constitutes interference light or an optical overlay and must be suppressed with a costly barrier or separation member. Since a significant part of the surrounding light from the same reflecting source also enters the reflected light beam, such light is received by the light receiver and can adversely affect its function. It is possible, however, to alleviate the last two problems with a polarization filter.